Fetal heart disease has an overall mortality of about 30%. Deaths from birth defects are the leading cause of infant mortality. Non-invasive echocardiographic/Doppler (echo/Doppler) heart rhythm and function assessment has become integral to the evaluation of high-risk fetuses with arrhythmias, conduction disease, structural cardiac defects, maternal SSA/SSB antibody exposure, hydrops fetalis, and other in utero conditions. While fetal Magnetocardiography (fMCG) is the best means presently for directly evaluating fetal cardiac rhythm and intervals, it is not readily available to clinicians. Assessment of accuracy, capabilities, and limitations of echo/Doppler-derived rhythm measurements is needed. No direct side-by-side cardiac interval validations have been performed between echo/Doppler and fMCG in a healthy fetal population, or in fetuses with disease. This is despite several ongoing studies using echo/Doppler interval measurement in the clinical stratification of fetuses with arrhythmias and SSA/Ro or SSB/La exposure. In 1997, we determined which echo/Doppler criteria could be used for electrocardiographic interval assessment in the neonate with supraventricular tachycardia (SVT) undergoing electrophysiologic testing. Direct electrocardiographic (EGG) and echo/Doppler measurements were simultaneously obtained. Severe disturbances in diastolic function were observed and characterized during SVT. The purpose of this present study is 1) to validate the non-invasive mechanical measurement of cardiac intervals using fMCG comparisons in a healthy control fetal population as well as in fetal disease, and 2) to expand the potential utility of echo/Doppler in better assessing the fetus with heart disease by incorporating non-traditional measurement techniques utilizing knowledge gained from fMCG arrhythmia analysis. This may lead to further integration of rhythm and function assessments in the fetus using two different, yet complimentary technologies.